<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <style>
        *{
            margin: 0
        }
        div{
            width: 100px;
            height: 100px;
            border: 1px solid;
        }
        /*n: 0-正无穷*/
        div:nth-child(n){
            background: pink;
        }
    </style>
</head>
<body>
    <div>

    </div>
    <div>

    </div>
    <div>

    </div>
</body>
</html>